Peroxide process for producing N-phosphonomethylglycine

ABSTRACT

A process is provided for producing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid with a peroxide to form an intermediate N-phosphonomethyliminodiacetic acid-N-oxide. Thereafter, the N-phosphonomethyliminodiacetic acid-N-oxide is converted to N-phosphonomethylglycine by adding a catalytic amount of a metal selected from the group consisting of iron, zinc, aluminum, vanadium and copper, or a compound selected from the group consisting of water-soluble vanadium compounds, ferrous salts and cuprous salts.

This is a division of application Ser. No. 07/542,995 filed June 25,1990 now U.S. Pat. No. 5,043,475.

BACKGROUND OF THE INVENTION

This invention relates to a process for the preparation ofN-phosphonomethylglycine, and more particularly to the preparation ofN-phosphonomethylglycine by the conversion ofN-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine usingperoxides.

N-Phosphonomethylglycine, known also by its common name glyphosate, is ahighly effective, commercially important, phytotoxicant useful incontrolling a large variety of weeds. It is applied to the foliage of avery broad spectrum of annual and perenial grasses and broadleaf plantsIndustrial uses include control of weeds along roadsides, waterways,transmission lines, in storage areas, and in other nonagriculturalareas. Usually, N-phosphonomethylglycine is formulated into herbicidalcompositions in the form of its various salts in solution, preferablywater.

U.S. Pat. No. 3,950,402 to Franz discloses a process for the productionof N-phosphonomethylglycine by forming an admixture ofN-phosphonomethyliminodiacetic acid, water, and a metallic catalystselected from the noble metals, heating the admixture to an elevatedtemperature (greater than 70° C. to avoid low yields) and contacting theadmixture with a free oxygen-containing gas.

U.S. Pat. No. 3,954,848 to Franz discloses a process for the productionof N-phosphonomethylglycine by reacting N-phosphonomethyliminodiaceticacid with an oxidizing agent, such as hydrogen peroxide, in an aqueousacidic medium in the presence of a strong acid at a temperature of fromabout 70° C. to about 100° C. It is disclosed that one should employ atleast 2 moles of the hydrogen peroxide for each mole of theN-phosphonomethyliminodiacetic acid, and preferably more.

Hungarian Patent Application No. 187,347 discloses a process for thepreparation of N-phosphonomethylglycine by the oxidation ofN-phosphonomethyliminodiacetic acid with peroxides using a catalyticamount of a metal compound selected from compounds of silver, iron, tin,lead, manganese or molybdenum. Molybdates are preferred. At temperatureslower than 80° C., usually a contaminated end product is obtained.Typically, the reaction is carried out at a temperature of above 80° C.and preferably above 100° C. at pressures exceeding atmospheric, whereinthe intermediate N-oxide is decomposed as rapidly as it forms. It isfurther disclosed that two mole equivalents of peroxide should be usedfor each mole of N-phosphonomethyliminodiacetic acid to obtainacceptable yields of N-phosphonomethylglycine.

Although satisfactory results are obtained by the above processes tomake N-phosphonomethylglycine, all of them suffer from one or moredisadvantages, such as the use of excessive amounts of peroxide, the useof strong mineral acids and/or reaction at elevated temperatures andpressures. Now, there is provided a process which producesN-phosphonomethylglycine in high yields at modest temperatures and atatmospheric pressure using substantially stoichiometric amounts ofperoxide to oxidize the N-phosphonomethyliminodiacetic acid to thedesire N-phosphonomethylglycine without using strong mineral acids, suchas hydrochloric acid or sulfuric acid.

SUMMARY OF THE INVENTION

These and other advantages are achieved in a process for producingN-phosphonomethylglycine by the oxidation ofN-phosphonomethyliminodiacetic acid with peroxide to form anintermediate N-phosphonomethyliminodiacetic acid-N-oxide, theimprovement which comprises adding a catalytic amount of a metalselected from the group consisting of iron, zinc, aluminum, vanadium andcopper, or a compound selected from the group consisting ofwater-soluble vanadium salts, ferrous salts, and cuprous salts.

DETAILED DESCRIPTION OF THE INVENTION

The intermediate, N-phosphonomethyliminodiacetic acid-N-oxide, is knownto those skilled in the art, and can be prepared by a number of methods.For example, the intermediate can be prepared by the teachings in U.S.Pat. No. 3,950,402 or U.S. Pat. No. 3,954,848, both to Franz. InHungarian Patent Application 187,347, the intermediate is formed fromN-phosphonomethyliminodiacetic acid using peroxides in the presence ofcompounds of silver, iron, tin, lead, manganese or molybdenum. In U.S.Pat. No. 4,062,669 to Franz, an N-organo-N-phosphonomethylglycine isoxidized with peroxide under acidic or basic conditions. Other methodsmay be known to those skilled in the art.

Any number of peroxides known to those skilled in the art can be used toprepare the N-phosphonomethyliminodiacetic acid-N-oxide. Suitableperoxides include hydrogen peroxide, performic acid, peracetic acid,perbenzoic acid, peroxytrifluoroacetic acid, benzoyl peroxide,benzenepersulfonic acid, and the like. Hydrogen peroxide is preferred,and it is advantageous to use hydrogen peroxide in the form of aconcentrated solution, say between about 30% and 60%.

In the process of the present invention, it is preferred to prepare theN-phosphonomethyliminodiacetic acid-N-oxide by contactingN-phosphonomethyliminodiacetic acid with a peroxide in the presence of acatalytic amount of a water-soluble molybdenum compound or awater-soluble tungsten compound. A water-soluble tungsten compound isespecially preferred.

The temperature of the process to prepare theN-phosphonomethyliminodiacetic acid-N-oxide can vary from as low asabout 20° C. to about 70° C. Although temperatures below about 20° C.can be used, such temperatures would require the use of cooling, and noadvantages are obtained. At temperatures above about 70° C., degradationof the N-phosphonomethyliminodiacetic acid-N-oxide is observed, whichaffects the final yield of the desire N-phosphonomethylglycine.Temperatures between about 20° C. and about 65° C. are preferred.

The salts of tungsten useful as catalysts to oxidize theN-phosphonomethyliminodiacetic acid to theN-phosphonomethyliminodiacetic acid-N-oxide are known to those skilledin the art. It is only necessary that the tungsten salts are soluble inthe reaction medium. Suitable tungsten compounds include tungstic acid,1,2-tungstophosphate, and barium tungstate. The alkali metal tungstates,such as sodium tungstate, potassium tungstate, and the like, providesatisfactory results, and the alkali metal tungstates are preferred.

The salts of molybdenum useful as catalysts to oxidize theN-phosphonomethyliminodiacetic acid to theN-phosphonomethyliminodiacetic acid-N-oxide are also known to thoseskilled in the art. It is only necessary that the molybdenum salts aresoluble in the reaction medium. Suitable molybdenum compounds includemolybdenum halides, such as molybdenyl trichloride and the like, alkalimetal molybdates, such as sodium molybdate and the like, or more complexsalts, such as the ammonium or alkali metal dimolybdates. Sodium andammonium molybdates are preferred.

The amount of catalyst to convert the N-phosphonomethyliminodiaceticacid to the intermediate N-phosphonomethyliminodiacetic acid-N-oxide canvary within wide limits. Concentrations between about 0.01 and about 5wt. % catalyst, based on the weight of theN-phosphonomethyliminodiacetic acid, provide satisfactory results. Atconcentrations of less than about 0.01 wt. % catalyst, the reaction isslow, and at concentrations greater than about 5 wt. %, no particularadvantage is seen, although such higher concentrations are not harmful.It is preferred to use between about 0.01 wt. % and about 1 wt. % basedon the weight of the N-phosphonomethyliminodiacetic acid.

In the process of the present invention, the amount of peroxide shouldbe the stoichiometric amount required to convert theN-phosphonomethyliminodiacetic acid to the intermediateN-phosphonomethyliminodiacetic acid-N-oxide. As will occur to thoseskilled in the art, when less than the stoichiometric amount of peroxideis used, the yield of the desired N-phosphonomethylglycine is lower. Aslight excess of peroxide can be used to insure a quantitativeconversion of the N-phosphonomethyliminodiacetic acid to theintermediate, but there is no advantage to using large excesses ofperoxide, and excesses of peroxide may be deleterious if water-solublecompounds, such as ferrous salts or cuprous salts, are used to convertthe intermediate to N-phosphonomethylglycine.

Regardless of the method used to prepare theN-phosphonomethyliminodiacetic acid-N-oxide from theN-phosphonomethyliminodiacetic acid, the intermediate is contacted witha catalytic amount of a substance selected from the group consisting ofiron metal, zinc metal, aluminum metal, vanadium metal or copper metal.Alternatively, a compound selected from the group consisting of thewater-soluble salts of a vanadium compound, ferrous salts, and cuproussalts can convert the intermediate N-phosphonomethyliminodiaceticacid-N-oxide to the desired N-phosphonomethylglycine. Suitable vanadiumcompounds that are soluble in the reaction mixture include vanadiumpentoxide, vanadium sulfate, vanadium chloride and the like. Suitablewater-soluble ferrous compounds that can be used in the process of thepresent invention include ferrous sulfate, ferrous halides, such asferrous chloride, ferrous bromide and the like. Suitable water-solublecuprous salts that can be used in the process of the present inventioninclude cuprous chloride, cuprous bromide, cuprous sulfate and the like.Of the water-soluble compounds, vanadium compounds are preferred, andvanadyl sulfate is especially preferred.

The amount of catalyst to convert the N-phosphonomethyliminodiaceticacid-N-oxide to N-phosphonomethylglycine depends upon the catalyst usedand the amount of peroxide in excess of that required to produce theintermediate from the N-phosphonomethyliminodiacetic acid. When metalssuch as iron, zinc, aluminum, vanadium and copper are used, the rate ofreaction to convert the intermediate N-phosphonomethyliminodiaceticacid-N-oxide to N-phosphonomethylglycine depends upon the surface areaof the metal present, and it is preferred to use from about 0.1 wt. % toabout 10 wt. % of the metal, based on the weight of theN-phosphonomethyliminodiacetic acid-N-oxide present. In addition, it ispreferred to use the metal in any form that provides a high surfacearea, for example, a wool, a powder or finely divided granules. However,when a water-soluble compound is used as a catalyst, the excess peroxidewill react with the water-soluble compound, and in addition to theamount of compound required to react with the excess peroxide, thereshould also be a sufficient amount of the water-soluble compound tocatalyze the reaction of the N-phosphonomethyliminodiacetic acid-N-oxideto N-phosphonomethylglycine. The amount of water-soluble compoundremaining after reaction with the peroxide to act as a catalyst shouldbe at least 0.005 wt. %, based on the amount of theN-phosphonomethyliminodiacetic acid-N-oxide. Excess water-solublecompound as high as 5%, or even higher, can be used, but there does notseem to be an advantage to using such higher concentrations for theconversion of the intermediate to N-phosphonomethylglycine, althoughsuch higher concentrations are not harmful. It is preferred to usebetween about 0.01 wt. % and about 2 wt. % of the water-solublecompound, based on the weight of the N-phosphonomethyliminodiaceticacid-N-oxide, after reaction with any excess peroxides.

The temperature required to convert the intermediateN-phosphonomethyliminodiacetic acid-N-oxide to the desiredN-phosphonomethylglycine can vary within wide limits. It is preferred toadd the catalyst at or near room temperature (about 20° C.) becausevigorous gas evolution frequently occurs, and the conversion ofN-phosphonomethyliminodiacetic acid-N-oxide to N-phosphonomethylglycineis exothermic. It is preferred to keep the reaction temperature belowabout 80° C. by cooling the reaction vessel or using a low catalystcharge. Temperatures above about 80° C. will provideN-phosphonomethylglycine, but some yield loss may occur.

The concentration of the N-phosphonomethyliminodiacetic acid as thestarting material can vary within wide limits in the process of thepresent invention. For example, an aqueous suspension containing up to50 wt. % N-phosphonomethyliminodiacetic acid can used. Higherconcentrations of the N-phosphonomethyliminodiacetic acid can be used,but it can present processing difficulties because of the thickness ofthe slurry. On the other hand, an aqueous solution of theN-phosphonomethyliminodiacetic acid containing about 5 wt. % of theN-phosphonomethyliminodiacetic acid can also be used. Lowerconcentrations can also be used, but it requires processing largevolumes of liquid in the process of the present invention. It ispreferred to use an aqueous slurry containing from about 20 wt. % toabout 40 wt. % of the N-phosphonomethyliminodiacetic acid.

The N-phosphonomethyliminodiacetic acid starting material can beprepared by methods known to those skilled in the art. For example, thismaterial can be produced by the reaction of formaldehyde, iminodiaceticacid and orthophosphorous acid in the presence of sulfuric acid.Although the N-phosphonomethyliminodiacetic acid mixture resulting fromthis reaction can be employed directly in the process of this invention,it is preferred to isolate the N-phosphonomethyliminodiacetic acid andthen employ it herein.

This invention is further illustrated by, but not limited to, thefollowing examples. Conversion is calculated by dividing the moles ofother compounds produced by the moles of startingN-phosphonomethyliminodiacetic acid and multiplying by 100. Selectivityis calculated by dividing the moles of N-phosphonomethylglycine producedby the moles of N-phosphonomethyliminodiacetic acid converted andmultiplying by 100.

EXAMPLE 1

This Example illustrates the process of the present invention using awater-soluble vanadium salt to convert the intermediateN-phosphonomethyliminodiacetic acid-N-oxide to N-phosphonomethylglycine.

(A) To a 100 ml round bottomed flask was added water (25 ml),N-phosphonomethyliminodiacetic acid (20 g), 47% hydrogen peroxide (7.1g) and sodium tungstate (0.05 g). The mixture was heated to 65° C. andmaintained at this temperature until a solution was obtained (about 58minutes), indicating the N-oxide was formed. The solution was thenallowed to cool to about 55° C. and stirred for an additional 30minutes.

(B) After cooling to room temperature, vanadyl sulfate (0.05 g, 29%water content) was added to the solution. After stirring for about 5minutes, the color of the solution changed from blue to light green. Gasevolution began with a slow exotherm. When the temperature reached about40° C., the exotherm greatly accelerated to 65° C. and cooling water wasapplied to maintain the solution at this temperature. The reactionmixture was allowed to cool to room temperature, the solids werefiltered, and the filtrate and solids were analyzed by HPLC. Theconversion of N-phosphonomethyliminodiacetic acid was 96.7%, and theselectivity to N-phosphonomethylglycine was 91.4%.

EXAMPLE 2

This Example illustrates the process of the present invention using awater-soluble ferrous salt to convert the intermediateN-phosphonomethyliminodiacetic acid-N-oxide to N-phosphonomethylglycine.

The procedure of Example 1 Step (A) was repeated. Then after thesolution was allowed to cool to room temperature, ferrous sulfate (0.02g) was added to the solution. Gas evolution was observed, and thetemperature of the solution rose to 65° C. Cooling water was applied tokeep the temperature below 70° C. The reaction mixture was allowed tocool to room temperature, the solids were filtered, and the filtrate andsolids were analyzed by HPLC. The conversion ofN-phosphonomethyliminodiacetic acid was 99.5%, and the selectivity toN-phosphonomethylglycine was 93.7%.

EXAMPLE 3

This Example illustrates the process of the present invention using zincmetal to convert N-phosphonomethyliminodiacetic acid-N-oxide toN-phosphonomethylglycine.

(A) To a 100 ml round bottomed glass flask was added water (37 ml),N-phosphonomethyliminodiacetic acid (14.0 g), 30% hydrogen peroxide (7.2g) and ammonium dimolybdate tetrahyrate (0.32 g). The mixture was heatedto 65° C. and maintained at this temperature until a solution wasobtained (about 30 minutes), indicating the N-oxide was formed. Thesolution was then allowed to cool to 45° C. and stirred for 50 minutes.

(B) After cooling to room temperature, zinc metal powder (0.4 g) wasadded to the solution. Vigorous gas evolution was observed, and thetemperature of the solution rose to 55° C. in about a 10-minute period.The reaction mixture was allowed to cool to room temperature, the solidswere filtered, and the filtrate and solids were analyzed by HPLC. Theconversion of N-phosphonomethyliminodiacetic acid was 91.0%, and theselectivity to N-phosphonomethylglycine was 93.8%.

EXAMPLE 4

This Example illustrates the use of copper metal, aluminum metal, and awater-soluble cuprous salt to convert the intermediateN-phosphonomethyliminodiacetic acid-N-oxide to N-phosphonomethylglycine.

The procedure of Example 3 Step (A) was repeated. After cooling to roomtemperature, small aliquots of the solution containing theN-phosphonomethyliminodiacetic acid-N-oxide were taken and placed in 25ml beakers. To one aliquot was added a copper penny. To another aliquotwas added aluminum foil. To a third aliquot was added a small amount ofcuprous chloride, and to a fourth aliquot was added vanadium metal. Inall cases, gas evolution was observed, indicating that the intermediateN-phosphonomethyliminodiacetic acid-N-oxide was converted toN-phosphonomethylglycine.

Although the invention has been described in terms of specifiedembodiments which are set forth in considerable detail, it should beunderstood that this is by way of illustration only, and thatalternative embodiments and operating techniques will become apparent tothose skilled in the art in view of the disclosure. Accordingly,modifications can be made without departing from the spirit of thedescribed invention.

What is claimed is:
 1. A process for producing N-phosphonomethylglycinewhich comprises oxidizing N-phosphonomethyliminodiacetic acid with aperoxide to form an intermediate N-phosphonomethyliminodiaceticacid-N-oxide, and then adding a catalytic amount of a water-solublevanadium compound to convert the intermediate toN-phosphonomethylglycine.
 2. A process of claim 1 wherein the amount ofcatalyst to convert the intermediate N-phosphonomethyliminodiaceticacid-N-oxide to N-phosphonomethylglycine is between about 0.005 wt. %and about 5 wt. %, based on the weight of theN-phosphonomethyliminodiacetic acid-N-oxide present.
 3. A process ofclaim 2 wherein the amount of catalyst is between about 0.01 wt. % andabout 2.0 wt. %.
 4. A process of claim 1 wherein the catalyst is vanadylsulfate.
 5. A process of any of claims 4 wherein the peroxide ishydrogen peroxide.